1. Field of the Invention
The invention relates to an exhaust gas purification system for an internal combustion engine that has a filter disposed in an exhaust passage for trapping particulate matter discharged from the engine and also relates to an exhaust gas purification method for such an internal combustion engine.
2. Description of the Related Art
Conventionally, as an exhaust gas purification system applied to an internal combustion engine, such as a diesel engine for a vehicle, a system is known which includes a filter disposed in the exhaust passage for trapping particulate matter (PM) discharged from the engine. In such an exhaust gas purification system, the exhaust backpressure increases as the amount of accumulated PM trapped by the filter increases and invites problems such as deterioration of fuel economy. Thus, the pressure differential between upstream and downstream portions of the exhaust passage relative to the filter is detected to estimate the amount of accumulated PM. If the amount of accumulated PM is excessive, the PM is removed, e.g., burned, to regenerate the filter. One method for regenerating the filter that is widely used uses an oxidation catalyst to lower the oxidation temperature of the PM and to oxidize the PM using the exhaust heat from the engine. The use of the oxidation catalyst provides an advantage such that the filter is continuously regenerated without receiving any energy from the outside.
However, if the engine is operating under conditions where the exhaust temperature is low, such as when the engine is idling and/or under a low load, for an extended period, the PM is not oxidized even though the PM continues to accumulate. This situation can invite deterioration of fuel economy resulting from the increase of the exhaust backpressure and/or abnormal burning resulting from the trapping of excessive PM. An exhaust gas purification system thus is proposed in which an exhaust throttle valve is disposed downstream of the filter and an opening amount of the exhaust throttle valve is controlled to maintain the exhaust temperature in a predetermined regeneration temperature range when the filter is regenerated (for example, see JP-A-4-81513). Another exhaust gas purification system is also proposed in which fuel injection times are controlled, in addition to the control of the exhaust throttle valve, to maintain the exhaust temperature in the predetermined regeneration temperature range when the filter is regenerated (for example, see JP-A-2005-76604). Maintaining the filter in the regeneration temperature range as discussed above expedites the oxidization of the PM and accelerates the regeneration speed of the filter.
Meanwhile, the exhaust gas purification systems described above have the following drawbacks when the exhaust throttle valve malfunctions, because the systems control the exhaust temperature by controlling the opening amount of the exhaust throttle valve. That is, if the opening amount of the exhaust throttle valve is not reduced to a desired opening amount under the filter regeneration condition, the exhaust temperature does not increase sufficiently and the oxidization of the PM trapped by the filter does not proceed adequately. In this connection, the regeneration speed of the filter is estimated erroneously, and the filter regeneration may be stopped prematurely. Thus, a significant portion of the accumulated PM remains trapped in the filter. If the PM remains, the exhaust backpressure may increase and adversely affect fuel economy. Also, if an excessive amount of PM is trapped, the filter may melt as a result of abnormal burning of the PM.